Bought a previously abused 4R55E 4speed 1999 Ford Ranger XLT 3.0l V6 flex fuel with 156k miles. Have had to do a ton of work but finally got 'er running. But suddenly VSS and speedometer read zero until around 20mph, when it suddenly catches up and transmission juts hard into the next gear. Sometimes it's fine but most of the time now this happens. Replaced VSS/abs sensor on rear differential even replaced pigtail. Checked wiring through gem and dash. Replaced GEM. But no luck. No other issues, no codes.

Could failing internal rear diff parts or filthy rear diff fluid be causing the vss not to read at slow speeds?